A FET (Field Effect Transistor) or a vacuum tube is used as an impedance converter of a capacitor microphone, because impedance between a vibrating plate and a fixed electrode placed and faced to each other in a capacitor microphone capsule is remarkably high.
When a vacuum tube is used as an impedance converter of a capacitor microphone, exclusive power supplies generally need to be prepared. A heater power supply for heating a heater and a plate power supply for flowing a plate current are necessary for operating the vacuum tube.
Generally, a vacuum tube which is used as an impedance converter needs a heater power supply of 6.3V, 0.35 A and a plate power supply of 120V, 10 mA. Therefore, since a general 48V phantom power supply cannot afford to supply that power, the exclusive power supplies are used.
A dry battery powered vacuum tube which has small power consumption is in existence. For example, type no. 6418 of a directly heated pentode vacuum tube of Raytheon company meets the requirement. This vacuum tube has been developed for a hearing aid and can be operated by a heater power supply of 1.25V, 10 mA and a plate power supply of 30V, 0.24 mA.
A vacuum tube which has as small the power consumption as the above-described power can be operated by the general 48V phantom power supply. However, when the heater power of 1.25V, 10 mA is directly stabilized from the phantom power by a zener diode or the like, the voltage supplied from the phantom power supply is decreased because the current of 10 mA flows to the heater of the vacuum tube.
For example, when 10 mA flows to the heater of the vacuum tube, the voltage supplied to the microphone from the phantom power supply decreases the voltage of 34V (5 mA×6.8 kΩ) and the voltage supplied to the microphone results in 14V. In this condition, the power supplied to the plate of the vacuum tube is insufficient and the vacuum tube cannot be operated.
Therefore, it is contemplated that a down-converter which converts a higher voltage to a lower voltage can be used. That is, a larger current is obtained from a smaller current by the down-converter. However, following problems will occur.
One of the problems will described hereinafter. Many converting methods for down converters are available. However, the pulse width modulation method, such as used in type no. MAX1733 of Low-Voltage Step-Down DC-DC Converter of Maxim Integrated Products, is generally used.
Since in the converter using the pulse width modulation method, the input voltage of the converter is switched on and off, large noise is generated because of the switching. The noise enters to a supply voltage, and then, the noise increases a microphone noise.
Further, since an inductor is used in the pulse width modulation converter, an external magnetic field is generated by the current with noise flowing in the inductor and the external magnetic field is magnetically coupled with an output transformer of a microphone, whereby the microphone noise is generated.
The other problem is that the down-converter which can convert 10V to 1.25V is rarely-offered in markets, because it's application is limited.